Adequate removal of rust from metal surfaces in preparation for the application of paint or other protective coatings is a long standing problem. Mechanical cleaning techniques such as sand blasting, wire brush scrubbing, etc. are messy and time consuming. Previous attempts to chemically clean rusty surfaces have not been entirely satisfactory.
One particularly difficult type of metal surface to clean is the irregular surfaces found on ships, i.e., high-temperature valves, pipes, and the like. Frequently, the only cleaning method feasible is the lengthy and tiresome process of wire brushing the surface to be cleaned and then subsequently applying a solution of a wetting agent mixed with a cleaning agent to the metal surface. Such a technique suffers from the difficulty of keeping the cleaning fluid in contact with the surface to be cleaned, such as overhead objects, as well as the subsequent disposal of liquid wastes. Additionally, these solutions are often toxic, non-economical, and require large volumes of water for washing purposes.
Previous attempts to chemically remove rust have involved the use of chemicals such as inhibited hydrochloric acid, ethylenediaminetetraacetic acid (EDTA), EDTA/citric acid, etc. More recently it has been suggested that a paste of water-soluble polymers such as polyvinylpyrrolidone (PVP) and a chelating agent such as EDTA be coated onto a rusty metal surface to be cleaned. After application the paste is said to harden into a thick crust which encapsulates the rust and may be peeled from the cleaned surface and disposed of as solid waste. This process is more fully described in U.S. Pat. No. 4,325,744.